Short circuits can occur in all electric power systems at all voltage levels.
The consequences of a short circuit are a high current and a voltage drop in large parts of the network. The short-circuiting current also causes mechanical and thermal strain on all apparatus and equipment upstream of the fault location. In most cases an arc occurs in the fault location which, in combination with the high short-circuiting current, produces damages at the fault location that demand repairs but also incur risk of personal injury.
The consequences of short circuits depend on the duration of the short circuit, the amplitude of the current and the level of the voltage drop.
In order to minimise the consequences of a short circuit, its duration must be minimised.
When a short circuit occurs the breaker closest upstream of the fault shall disconnect the short circuit. The duration of a short circuit depends on how quickly it can be detected and how long is required to disconnect it.
The consequences of a short circuit are also dependent on where it occurs in an electrical system.
Short circuits can be divided into three groups as regards where the fault occurs:    A. Short circuit on the supply side    B. Short circuit in the switchgear installation.    C. Short circuit on an out-going group.
Regardless of where the fault occurs, all apparatus and components must be dimensioned for a short circuit producing the highest short-circuiting current. If the short-circuiting current exceeds the rated data existing on the apparatus, the system must be altered so that the short-circuiting current is kept at the levels the switchgear installation and breakers can handle. A usual measure is to divide the supply side between two transformers and busbars. Nowadays 64 kA is the highest short-circuiting current permitted for breakers and switchgear installations.
If a short circuit occurs in a switchgear installation, the switchgear installation and breaker on the supply side must be dimensioned for the estimated short-circuiting current. The switchgear installation must also be dimensioned so that it can withstand the pressure increase that occurs in the event of an arc in the switchgear installation.
Extremely large quantities of energy in the form of heat and radiation are released in arcs caused by large leakages. This energy gives rise to pressure increases in enclosed switchgear installations with restricted space, for instance, which may burst the enclosure. Such switchgear installations must therefore be equipped with bulky relief openings through which the hot gases can flow out. The high arc temperatures also cause the material in conductors and coupling equipment to melt, or even to be vaporised. Combustible organic material may also be ignited when it is subjected to the high temperatures and intense radiation of the arc. The arc also gives rise to toxic gases through decomposition of air (NOx) and vaporisation of metals. It is therefore usual for such switchgear installations to be provided with arrangements for pressure relief in the form of evacuation channels hatches that open automatically, etc. Such switchgear installations therefore become space consuming and expensive.
There has long been a great need in industry, first of all to prevent the occurrence of arcs and secondly to minimise the duration of arcs. Material damage as a result of the heat and pressure increase built up during the existence of the arc can therefore be reduced. The risk of personal injury and poisoning are also reduced.
It is therefore a matter of urgency to prevent as far as possible the occurrence of arcs and, when they do occur, to endeavour to extinguish them as quickly as possible. Breakers are normally used for this purpose. Besides being expensive, these breakers have the drawback of reacting relatively slowly. On the cable from the transformer, which supplies the switchgear from the external network, the breaking time is in the order of 200 ms because of the selectivity requirement of the relay protection. In the cables from the switchgear to the loads the breakers are normally capable of breaking more quickly, ca 40 ms. A breaking time of 200 ms is too long to effectively prevent damage as a result of arcs being formed. When an arc appears in the switchgear installation itself, therefore, other measures must be taken to quickly extinguish the arc. A branch cable to earth from the feeder cable from the transformer is sometimes arranged for this purpose and provided with a normally open closing contact, which, in the event of a short-circuiting fault, connects the feeder cable to earth so that the arc is extinguished. The operating time for such a closing contact is normally around 20 ms.
For practical reasons, this method is not used for arcs appearing in the system after the switchgear installation. Neither is the same saving in time so significant in relation to the time it takes for the breakers in these cables to be activated, i.e. ca. 40 ms.
An operating time of 20 ms for a closing contact is often sufficient to prevent the arc from causing major damage. However, even during this time a certain amount of havoc can occur and it would be desirable to be able to achieve even shorter operating times.
Upon short-circuiting in an out-going group the breaker on the out-going group must be dimensioned to be able to break the short-circuiting current that occurs and the switchgear installation must be dimensioned to handle the same short-circuiting current.
A particular problem exists in systems where the switchgear installation receives currents from two or more feeder cables. This is becoming increasingly usual in industry. One reason is that a strong net is desirable, i.e. with high short-circuiting effect, in order to avoid voltage fluctuations when loads with high output are connected. In most cases this means the start of large motors. A strong network is achieved by increasing the number of incoming supplies to the switchgear installation or by connecting together the busbar systems of two or more switchgear installations.
Another case where several feeder cables are used is when a local generator is connected to the switchgear installation. This is becoming increasingly usual. Many industries make use of waste energy from their own processes to generate electricity. This is usually supplied to the public network after step-up transformation. By instead supplying their own consumption the cost of a transformer is saved and the system becomes strong.
The principal drawback of connecting switchgear installations together or having several supplies to a switchgear installation from the transformer and/or generator is the high short-circuiting currents that appear in the medium-voltage system. The leakage in the medium-voltage system consists of the sum of the currents from the various feeder cables and the rated data for switchgear installation and breaker may be exceeded. This can nowadays been remedied by arranging a current limiter between the various switchgear installations or between the various parts of a switchgear installation with several feeders. However, this is an extremely expensive solution and entails other complications.